The present invention relates generally to traffic control, and specifically to a microwave vehicle detector for detecting traffic movement in a predetermined direction.
The most widespread form of vehicle traffic control used in urban areas today is provided by a network of traffic lights. In view of the fact that the frequency of traffic at individual intersections will generally vary during different times of the day, it would be desirable to adjust the sequencing operation of these traffic lights in response to such traffic variations. In this regard, many traffic systems currently use computers so that the sequencing operation of the traffic lights may be varied under programmed control. However, the effectiveness of such programmed control schemes is limited because traffic patterns are subject to change, particularly in areas which are experiencing rapid development.
Accordingly, some traffic systems have additionally employed underground loops at various intersections in order to provide on the spot feedback to a centralized controller of the traffic conditions at these intersections. In this regard, one or more loops will be buried at different points in the pavement to detect whether traffic is present, and how much traffic is present if more than one loop is used. However, these loops are subject to damage due to seasonal temperature changes or heavy traffic. In the event that replacement is required, then the surrounding pavement will also have to be cut out and replaced. This will not only be expensive, but this replacement will inevitably disrupt traffic at that location.
The use of an ultrasonic sensor has also been suggested as an alternative to the use of underground loops. However, outdoor use of an ultrasonic sensor is problematic in that temperature and humidity will affect the speed of sound. Additionally, the sensor may have difficulty in differentiating between the sound that has been transmitted by the sensor and other sources of ultrasonic sound.
More recently, a microwave based detector device has been used for detecting vehicle movement, namely the TC-20 vehicle detector manufactured by the assignee of the present invention. However, the TC-20 detector had a relatively limited range and discrimination capability, and provided only an analog output signal.
Accordingly, it is a principal objective of the present invention to provide a microwave based vehicle detector which has a substantially increased range of detection without increasing the power to the microwave transceiver.
More specifically, it is an objective of the present invention to provide a microwave vehicle detector which not only detects and analyzes traffic movement, but also provides a variety of buffered outputs that can be used by external devices, such as a monitoring computer or traffic signs.
It is another objective of the present invention to provide a microwave vehicle detector which is capable of determining the speed of the vehicles being monitored and enabling the displaying this information in a convenient fashion.
In order to the achieve the foregoing objectives, the present invention provides a microwave vehicle detector which includes a transceiver for transmitting microwave energy into a desired target area and for receiving a reflection of this microwave energy from vehicle movement in this target area. The transceiver includes first and second receiver diodes for enabling the microwave vehicle detector to determine the direction of vehicle movement relative to the position of the microwave vehicle detector. In this regard, these diodes are balanced, such that a predetermined phase difference will be produced between the doppler shift signals generated by these diodes. This phase difference is analyzed by a microprocessor to determine the speed and direction of the vehicle being detected.
The microwave vehicle detector also includes signal shaping circuitry for transforming the doppler shift signals generated by the detector diodes into square wave signals suitable for a type of pattern recognition analysis by the microprocessor. A relay control and driver circuit is also provided for causing a normally activated relay to become deactivated in response to the detection of vehicle movement through the target area in the predetermined direction selected for the microwave vehicle detector. The microprocessor also generates a number of output signals which may be used to indicate the speed of the target vehicle. These output speed signals may also be used in intelligent vehicle and highway systems and/or control the messages displayed on roadway message signs, such as work crew signs.
The present invention also includes a monitor circuit for causing the relay to become deactivated in response to a predetermined change in the bias level of either the first or second detector diodes. Specifically, the monitor circuit includes a voltage sensitive switching circuit connected to each of the detector diodes, and a controlled conduction device which is responsive to these voltage sensitive switching circuits. The output from the controlled conduction device is connected to the relay control and driver circuit, such that the monitor circuit will tell the microprocessor to trigger the deactivation of the relay when the bias level of either of the detector diodes has past beyond certain threshold levels.
When the microprocessor triggers the deactivation of the relay, the relay will be deactivated until the bias condition is corrected. In contrast, when vehicle movement triggers the deactivation of the relay, the relay will automatically be reactivated by the microwave vehicle detector after a predetermined time duration. This difference in deactivation times will enable a traffic controller to discern whether or not the microwave vehicle detector is operational.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :